1. Field of the Invention
The present invention relates to a communication system, a communication apparatus, and a communication method, and more particularly, to a communication control technique for suppressing interference with another communication system.
2. Description of the Related Art
A scheme of wirelessly connecting closeby apparatuses is called a WPAN, and is defined separately from a WLAN.
ECMA International as a standards organization has defined the physical layer and MAC layer specification as the ECMA-368 standard assuming a WPAN that uses a UWB (Ultra Wide Band) communication scheme. The ECMA-368 standard is also called the WiMedia standard. As a protocol operating at the upper layer of the ECMA-368 standard, the wireless USB (Universal Serial Bus) standards and the like have been defined.
In order to prevent collision of wireless frames which occurs when a plurality of wireless terminals simultaneously transmit wireless frames, the UWB communication scheme controls a timing at which each wireless terminal accesses another wireless terminal. This control scheme is defined by a Media Access Control (MAC) protocol, and is implemented by transmitting/receiving beacons.
In the UWB communication scheme, interference occurs between wireless communication systems due to frequency sharing. It is therefore important to suppress the interference for commercial use.
To suppress occurrence of interference between wireless communication systems, it is effective to decrease the transmission power of each wireless terminal in each wireless communication system.
Conventionally, various schemes of controlling transmission power for power savings have been proposed in a wireless communication system.
For example, Japanese Patent Laid-Open No. 2005-328231 has proposed the following scheme. That is, the transmission power of a wireless terminal is gradually increased, and a transmission power level upon reception of a link establishment completion signal from an access point is set in subsequent data communication.
Furthermore, Japanese Patent No. 3762872 has proposed the following scheme. That is, a control station notifies a terminal station of the lower limit expected value of reception power at which the control station can receive a signal and the terminal station sets the transmission power of itself with respect to the lower limit expected value.
However, in the above patent references, the transmission power is controlled by determining the transmission power level necessary for maintaining communication with other wireless terminals within a wireless communication system of a self-terminal. That is, the above schemes are not aimed at actively avoiding “interference” with another communication system. When the terminals communicate at the controlled transmission power, it is not always possible to avoid “interference” with another wireless communication system.